2 1 6 INFLUENCE OF CORRELATION AND EXTERNAL STIMULI 



conform in the first instance entirely with those of the foliage-leaves, and 

 it is the appearance of the sporangia which induces their different develop- 

 ment from that of the primordia of the foliage-leaves. Comparative 

 observations have made it certain that such a correlation exists l . The 

 experimental proof which has been furnished in the case of Onoclea 

 Struthiopteris bears upon this point 2 . By the removal of the foliage- 

 leaves the formation of sporangia can be entirely or partially hindered, 

 and the primordia of the sporophylls can be forced into development 

 as foliage-leaves 3 . 



The same is the case in bud-scales. Those which serve as pro- 

 tective organs to the bud during its resting period differ from the foliage- 

 leaves of the plant to which they belong only to an insignificant extent 

 in size and form ; they have, even where the foliage-leaves are much 

 segmented, only the form of simple scales, a form which for their work 

 as ' covers ' to the bud is the most favourable. There are three methods 

 of transformation of the primordia of the foliage-leaves to bud-scales : 

 usually the bud-scale is formed out of the basal part of the leaf- 

 primordium, the leaf-base, whilst the primordium of the blade is arrested 

 and the petiole is not developed ; or the primordium of the blade is 

 arrested and the stipules develop to bud-scales ; or finally the whole 

 leaf is transformed into a bud-scale. In all three forms it is possible 

 to show that the transformation can be hindered. This happens where 

 buds which are destined for a succeeding year develop into shoots in the 

 year of their formation, and according to the moment at which this takes 

 place will the bud-covers become foliage-leaves, or, if the transformation 

 had already begun, intermediate forms between bud-scales and foliage- 

 leaves will be found. 



As a last illustration I may mention that one can often hinder the 

 transformation of the leaflets of the pea into tendrils if one removes 

 the other leaves. 



All these examples, to which many others might be added, show that 

 correlation plays an important part in the formation of organs, and this 



1 See what 1 say on this subject in my Vergleichende Entwicklungsgeschichte der Pflanzenorgane.' 

 " Goebel, Uber kunstliche Vevgrunung von Farnsporophyllen, in Ber. der deutsch. botan. 

 Gesellsch. v (1887). Figures will be found in Annals of Botany vi, plate xxii. 



3 In Selaginella also a correlation is observable between the formation of sporangia and the 

 formation of the leaves (and the whole shoot clad with sporophylls). I have already proved that in 

 Selaginella Lyalli the sporangiferous spikes, which differ from the vegetative shoots in the form of 

 the leaves, may develop into vegetative shoots if the formation of sporangia be arrested. Behrens 

 has recently shown that this regularly happens if sporangiferous spikes are used as cuttings, and he 

 concludes that the growing out of the apex of the spike causes the suppression of the sporangia. 

 (See Behrens, Uber Regeneration bei Selaginella, in Flora, Erg.-Bd. 1897, p. 159.} To my mind a 

 better explanation is that the severance of the spike causes a disturbance in the formation of the 

 sporangia which permits of the further vegetative development. 



